In a blood extra corporeal circulation therapy using a blood line, a continuous monitoring of the pressure in the blood line is of necessity as an alerting system against line blockage. A prevailing method for such monitoring is a method to monitor the pressure via an air layer using a drip chamber as a pressure receiving aperture part in order to avoid a blood counter flow toward the pressure measuring part and to remove air in the blood line. However, in such a method, blood coagulation is likely to occur due to the activation of coagulation factors at the interface between the air and blood in the drip chamber, resulting in a major cause of line blockage.
In particular, since a continuous blood purification method (CHDF, CHF, CHD), which involves a low flow rate of blood compared to general dialyses, is continuously operated for more than about 24 hours and is often used for patients with hemorrhagic lesion to whom administration of blood anticoagulant is undesirable, coagulation factors tend to be activated thereby causing a condition in which blood coagulation at the interface between blood and the air is likely to occur. Although the continuous blood purification is constantly controlled for nearly 24 hours mainly in an ICU (Intensive Care Unit), constant monitoring of the inner pressure of the circulating line is still important and is commonly performed. Nevertheless, there is constant danger of blood coagulation due to the contact with the air in the drip chamber. In view of these situations, conventionally, non-air-contact measurement techniques have been contrived as a method of measuring the pressure in an extra corporeal circulation blood line. JP,Y2, 4-1948 discloses a method of monitoring fluid pressure, in which a diaphragm for separating the air from blood was introduced into a drip chamber as a means for preventing the contact between the air and blood, and the pressure fluctuations of the air layer were monitored through the diaphragm. However, in this method, a problem existed in that when the chamber becomes a normal pressure for some reason, the diaphragm becomes an excessively stretched state disabling the pressure measurement thereafter.
On the other hand, there has been contrived a method, which is also a diaphragm method, but in which pressure is measured by placing a load cell into a direct and close contact with a diaphragm, instead of measuring pressure through an air layer. JP,A, 8-166301 discloses a method of measuring the inner pressure of a circulating line by using a pillow in an extra corporeal circulation line for similar purpose with a diaphragm and by placing a load cell in contact with the pillow. According to the aforementioned publication, it is made possible to accurately measure the pressure with a load cell by placing a holder cover which can maintain a predetermined space with respect to a pressure transfer plate as the means to prevent the changes in the contact area between a pillow diaphragm and the aforementioned plate. However, to detect negative pressure successfully, the pillow diaphragm is constantly applied with stress; and therefore the pillow diaphragm would deform when used for a long period of time thereby suffering a shift in zero point and degradation in pressure sensitivity. Further, in this method, since a polymer membrane is used as the material of the diaphragm, a creep deformation, which is characteristic of polymer materials, may occur, and therefore, this method is not suitable for measuring the pressure in a circulating line in a continuous and stable manner for a long period of time.